Technical Field
This disclosure relates to a sheet feeder, an image forming apparatus incorporating the sheet feeder, and an image forming system incorporating the sheet feeder.
Related Art
Various types of electrophotographic image forming apparatus are known to include a sheet feeder to feed sheets to an image forming device from a sheet loader on which a bundle of sheets are loaded. In the sheet feeder, an upper sheet placed on the bundle of sheets on the sheet loader is lifted by air blown from an air blowing device, so that the floating sheet is conveyed one by one by a conveying member such as an attraction belt. Such a sheet feeder has a sheet detection technique in which, when multiple upper sheets are lifted by air from the air blowing device, a sheet detection sensor such as a reflective optical sensor detects the side face of the multiple sheets, so as to move the sheet loader vertically (elevate and lower) according to the output value of the reflective optical sensor.
For example, a comparative sheet feeder includes a reflective optical sensor to detect multiple sheets in a range of from an upper face of a bundle of non floating sheets including sheets not floating in the air while the air blowing device blows air to the conveying member. Hereinafter, the range is referred to as a “sheet floating region”. The comparative sheet feeder further includes a lifting device to move the sheet loader up and down in a vertical direction and a controller to control the lifting device according to the output value of the sheet detection sensor.
The comparative sheet feeder detects the density of floating sheets in the sheet floating region (full or empty of floating sheets) according to the output value of the sheet detection sensor. When the number of floating sheets is decreased to a certain amount, the controller causes the sheet loader to elevate. By so doing, elevation of the bundle of floating sheets is controlled so as to float the specified number of sheets.
As the number of sheets loaded on the sheet loader decreases and approaches an empty state in which a single and last sheet remains, an interval of floating sheets increases to cause the space density of the floating sheets in the sheet floating region to become low. In this state, an uppermost sheet does not approach an attraction belt, and it is likely to cause sheet feed failure. When a remaining amount of sheets loaded on the sheet loader is less than a threshold value, the comparative sheet feeder sets a greater amount of sheets in the sheet loader than a regular amount of elevation. According to this configuration, the space density of floating sheets in the sheet floating region is made to be the specified value. Therefore, even if the remaining amount of sheets is small and a sheet feeding cycle is short, the comparative sheet feeder can cause the sheet to attract to the attraction belt by a subsequent sheet conveying timing.